Do nodes in EEVEE work the same as in Cycles?
Eeevee uses the same node structure as Cycles, and its aim is to make its materials as consistent as possible with Cycles materials. Ideally, you should be able to preview the result of your Cycles node structure by switching to Eevee or by using the Eevee-powered "Material Preview" viewport directly. 

This already works almost perfectly with the nowadays very common PBR texturing workflow that uses the Principled Shader and color/roughness/normal/displacement textures.

Here's an example material with Mix shaders, Fresnel inputs, Noise textures, Bump vector nodes, etc.: they all work in both engines and you can hardly see the difference in the final result.



Note: the wowen material in the rest of the monkey head is "Weavr" from Simon Thommes, which is also surprisingly consistent among render engines, despite it being quite complex.



However, not every node is present or is fully featured in both renderers equally. This goes both ways: the Shader to RGB node for non photo-realistic (NPR) renders is EEVEE only, as well as the Specular BSDF. 

Anisotropic, Holdout, Toon, Velvet, Hair and Principled Hair shaders only work with Cycles, as well as several input nodes: e.g. Particle Info, Bevel. 

This list may change: see "Node Support" in the Manual for detailed and updated information.



Note that you can have different "Material Output" nodes, one for each render engine, in the same material, in case you would like to tweak the materials independently (e.g. to make use of Cycles-only features in Cycles while faking them somehow in Eevee)